Viewing instruments, such as endoscopes, are generally well known in the art. Generally, an endoscope is a medical device for insertion into a body passageway or cavity that enables an operator to view and/or perform certain surgical procedures at a site inside a patient's body. As is known, endoscopes may be either rigid or flexible, and generally include a long tubular member equipped with, for example, some type of system for transmitting images to the user, and in some cases, a working channel for a surgical instrument. The endoscope has a proximal end that remains external to the patient, from which the operator can view the site and/or manipulate a surgical instrument, and a distal end having an endoscope tip for insertion into the body cavity of the patient.
Generally, these instruments employ some form of objective lens system, which focuses the image onto some form of image guide, such as a fiber optic bundle or relay lenses, thereby transmitting the images from inside the body cavity of the patient to the user's eye located at the proximal end of the endoscope, or to a camera likewise connected to the scope for subsequent display on a monitor and/or storage on an image capture device. Generally, these objective optical systems attempt to simultaneously maximize the field of view, maximize the image quality, provide telecentric image transmission to the image guide, and minimize the size and cost of the system.
For example, U.S. Pat. No. 4,354,734 to Nakahashi discloses an objective optical system with a telecentric design that has been very effective in providing a wide field of view in a compact, low-cost assembly. A number of retrofocal optical systems have been proposed, such as those described in U.S. Pat. No. 4,037,938 to Yamashita et al., U.S. Pat. No. 4,042,295 to Yamashita et al., U.S. Pat. No. 4,059,344 to Yamashita, U.S. Pat. No. 4,662,725 to Nisioka, and U.S. Pat. No. 6,256,155 to Nagaoka. However, all of these disclosures pertain to objective systems for endoscopes that have fixed viewing directions, and are not appropriate with endoscopes having a variable direction of view.
The operating principles of such a variable direction of view scope are described in U.S. Patent Application No. 2005/0054895 by Hoeg, et al., the specification of which is hereby incorporated herein by reference. Generally, such a scope has a view vector with an attendant view field that has at least two degrees of freedom. The first degree of freedom permits rotation of the view vector about the longitudinal axis of the endoscope's shaft, which allows the view vector to scan in a latitudinal direction, while the second degree of freedom permits rotation of the view vector about an axis perpendicular to the scope's longitudinal axis, which allows the view vector to scan in a longitudinal direction. In some cases, a third degree of freedom is also be available.
A number of such variable direction of view scopes have been proposed that use adjacent fixed and variable prisms to provide the variable direction of view, such as, for example, the designs disclosed in U.S. Pat. No. 3,880,148 to Kanehira et al., U.S. Pat. No. 4,697,577 to Forkner, U.S. Pat. No. 6,648,817 to Schara et al., German Patent DE 299 07 430, WIPO Publication No. WO 99/42028 by Hoeg, WIPO Publication No. WO 01/22865 by Ramsbottom.
A typical example of a basic dual reflector system is illustrated schematically in FIG. 1A. A pivotable reflector 10, usually a prism, reflects received light to a fixed reflector 12, also a prism, which further reflects the light into an optical train 14 for transmission to the viewer. In this way, the reflectors 10, 12, define an optical path comprising three segments 16, 18, 20. A view vector 22 exists in coincidence with the first optical path segment 16. By rotating the pivotable prism 10 about a rotational axis 24 coincident with the second optical path segment 18, the view vector 22 can be swept around in a plane normal to the rotational axis 24 (i.e., normal to the page). Even though this design is optimally compact, the use of only the rotating and fixed prisms 10, 12 results in an unacceptably small field of the view and is not telecentric.
Therefore, improved versions of the basic dual reflector design, employing additional optical mechanisms for improving the field of view, have been proposed. An example of such a system is shown in FIG. 1B. As illustrated, the design involves a simple retrofocus arrangement having a negative lens 26, an aperture stop 28 placed on the reflective face of the fixed prism 12, and a positive lens group 30. While this design provides an improved field of view, it is still not telecentric, does not provide sufficient chromatic and geometric correction, and is not optimally compact, as evidenced by the increased size of the pivotable prism 10. Additionally, the increased prism size also causes the scanning range to be limited, as the rotating prism 10 would be obstructed by the lens group 30. Finally, the reflecting surface (i.e., hypotenuse) of the fixed prism 12 is not the optimal place for the aperture stop 28.
Therefore, a continuous challenge presented by these systems is producing a suitable objective optical system that adequately accommodates this sort of dual reflector design. At the same time, there remains, in addition to the performance of the particular objective system, the ever-present desire to minimize the space required by the optics, including both the rotating and non-rotating prisms, as well as any other elements employed, as it is generally desired to produce scope diameters that are as small as possible in order to facilitate insertion and retraction. Because a dual prism design, such as those noted above, entails the use of two prisms positioned side-by-side transverse to the longitudinal axis of the scope, the scope diameter is usually somewhat large.
Therefore, it is desirable to design the system in such a way that the size of the optics can be minimized, while still providing the advantages of telecentricity, a large scanning range, a large field of view, and good image quality in a cost-effective manner. To date, this has been difficult to accomplish, as these interests often conflict. For example, decreasing the size of the optical elements typically reduces the amount of light admitted by the system and adversely affects the image brightness. As another example, increasing the field of view typically exacerbates optical aberrations and degrades image quality.
One of many critical design parameters in the optical system of such instruments is the entrance pupil, which is the location where the diameter of the light beam is minimal. This is also the location where an aperture stop can be optimally located to best condition the image and control image brightness and other image quality parameters. Most of the proposed designs noted above do not even mention the existence of an entrance pupil or aperture stop anywhere in the optical systems, while the design of Ramsbottom, for instance, apparently has the entrance pupil and accompanying aperture stop at the reflective face of the fixed reflector. This is not ideal, as this location of the aperture stop negatively affects both system size and performance—the system should be designed to accommodate larger diameter light flow on either side of it.
What is desired, therefore, is an optical system for a variable direction of view instrument that maximizes the field of view. What is further desired is an optical system for a variable direction of view instrument that maximizes the image quality and provides telecentric image transmission to the image guide. What is also desired is an optical system for a variable direction of view instrument that minimizes both the size and cost of the instrument.